Why does my itunes keep putting songs on my iphone

Everytime i want to add one song from my itunes library to my iphone, it puts all the music on there. I have the manually manage music selection on. I add one song, the top of the itunes says "preparing to update" then it adds 1000 songs that i don't want on there. Literally. After i add one song, i delete another thousand to make room on my phone. Its a major pain in the rear end. Why is this happening?